oxidation behavior of matrix graphite and its effect on

Research ArticleOxidation Behavior of Matrix Graphite and Its Effect onCompressive Strength

Xiangwen Zhou,1 Cristian I. Contescu,2 Xi Zhao,1 Zhenming Lu,1 Jie Zhang,1 Yutai Katoh,2

Yanli Wang,2 Bing Liu,1 Yaping Tang,1 and Chunhe Tang1

1 Institute of Nuclear and New Energy Technology of Tsinghua University, Collaborative Innovation Center ofAdvanced Nuclear Energy Technology, The Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education,Beijing 100084, China2Oak Ridge National Laboratory, UT-Battelle Inc., P.O. Box 2008, Oak Ridge, TN 37831, USA

Correspondence should be addressed to Xiangwen Zhou; [email protected]

Received 22 December 2016; Revised 15 June 2017; Accepted 4 July 2017; Published 1 August 2017

Academic Editor: Eugenijus Uspuras

Copyright © 2017 Xiangwen Zhou et al.This is an open access article distributed under theCreative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Matrix graphite (MG) with incompletely graphitized binder used in high-temperature gas-cooled reactors (HTGRs) is commonlysuspected to exhibit lower oxidation resistance in air. In order to reveal the oxidation performance, the oxidation behavior ofnewly developed A3-3 MG at the temperature range from 500 to 950∘C in air was studied and the effect of oxidation on thecompressive strength of oxidized MG specimens was characterized. Results show that temperature has a significant influence onthe oxidation behavior of MG. The transition temperature between Regimes I and II is ∼700∘C and the activation energy (𝐸

𝑎) in

Regime I is around 185 kJ/mol, a little lower than that of nuclear graphite, which indicates MG is more vulnerable to oxidation.Oxidation at 550∘C causes more damage to compressive strength of MG than oxidation at 900∘C. Comparing with the strengthof pristine MG specimens, the rate of compressive strength loss is 77.3% after oxidation at 550∘C and only 12.5% for oxidation at900∘C.Microstructure images of SEM and porositymeasurement byMercury Porosimetry indicate that the significant compressivestrength loss ofMGoxidized at 550∘Cmaybe attributed to both the uniformpore formation throughout the bulk and the preferentialoxidation of the binder.

1. Introduction

Graphite offers numerous advantages for in-core nuclearapplications because of its thermomechanical properties andchemical inertness in nonoxidizing environments.Therefore,it is widely used in HTGRs as the moderating, reflector,structural, and fuel element matrix materials [1–3]. However,graphite is easily oxidized by air at temperatures greaterthan 450∘C [4]. In low probability, yet imaginable off-normalevents, air orwater ingress accidentswould cause fast graphitecorrosion that may affect the core and fuel integrity inHTGRs. Many studies related to the oxidation behavior andits impact on the mechanical properties of nuclear graphitematerials have been reported in recent years [5–8]. Contescu[8] studied the effect of the oxidation temperature on thecompressive strength of PCEA graphite. Samples in the direc-tion of grain were oxidized in air at two temperatures (600

and 700∘C) and three levels of weight loss. Results showedthat oxidation at 600∘Cwas more damaging on strength thanoxidation at 700∘C, at comparable levels of weight loss whichwas due to the differences in the distribution of oxidationlayer and mechanism of development of porosity. Whereasa block reactor core consists mainly of highly graphitizednuclear graphite and contains only a small amount of fuelelement matrix graphite (MG); the active pebble-bed coreconsists of a large part of fuel element matrix graphite [9].The MG contains around 10% of incompletely graphitizedresin-derived carbon because of temperature limit restric-tion (<2000∘C) during the fabrication process of pebblefuel elements [3]. Because of its incompletely graphitizedbinder content, the activation energy of historicmatrix-gradegraphitic materials is lower than that of most modern nucleargraphite [9–12]. The activation energies of filler and binderfor A3-27 were reported separately by Moormann et al. [9].

HindawiScience and Technology of Nuclear InstallationsVolume 2017, Article ID 4275375, 6 pageshttps://doi.org/10.1155/2017/4275375

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2 Science and Technology of Nuclear Installations

The activation energy of the filler (165 kJ/mol) is largerthan that of the binder (123 kJ/mol). Lee et al. studied theoxidation rate of graphitic matrix material GKrS producedby ORNL thermogravimetrically with temperatures from873 to 1873K [10]. The activation energy for GKrS wasdetermined to be 111.5 kJ/mol in the kinetic regime, from 873to 1023K. It is supposed that the lower activation energy ofGKrS matrix graphite could be attributed to the preferentialoxidation of the binder phase in the kinetic regime [10].Manyresearchers have observed the preferential binder oxidationin their studies as well [11–13]. The preferential oxidation ofbinder often has a significant influence on the degradationof mechanical properties of graphite in the kinetic regimeof graphite oxidation that may lead to severe consequencessuch as failure of pebble fuel elements and subsequent releaseof large amounts of radioactive fission products. Comparingwith the large amount of reported researches on the oxidationbehavior of nuclear graphite, oxidation experiments andbasic kinetic data for matrix-grade graphite in air are sparsebecause the historic grades of A3-3 or A3-27 from Germanyare no longer commercially available and modern candidategrades are still being developed and difficult to obtain [10]. Inthis study, the A3-3 MG composing approximately 71% natu-ral flake graphite, 18% artificial graphite, and 11% phenol resinwas newly developed and manufactured in INET, TsinghuaUniversity of Beijing, China [3]. Chinese domestic producersproduced all the raw materials used in the preparation of thenewly developed A3-3 MG. The oxidation performance ofthe MG is very crucial for the integrity evaluation and safetyanalysis of pebble fuel elements in normal and off-normalconditions. In order to reveal the oxidation performanceof the MG, the oxidation behavior of MG in air in thetemperature range from 500∘C to 950∘C was studied, andthe effect of oxidation at two typical temperatures on thecompressive strength of MG was characterized.

2. Experimental

2.1. MG Specimens. All MG specimens covered by thisreport were machined from the MG pebbles manufacturedby the Institute of Nuclear and New Energy Technology(INET) [3]. In order to meet the specimen requirementsof standard test method for compressive strength of carbonand graphite (ASTM C-695) [14], a compromise had to bemade to machine the specimens into cylindrical shape with2 : 1 ratio between length (25.4mm) and diameter (12.7mm).This exceeds the recommended minimum specimen size of9.5mm diameter and 19mm length for compressive strengthtest. Meanwhile, as the maximum grain size in MG is0.16mm, the specimens with diameter of 12.7mm were incompliance with the requirement that the diameter of testspecimens for compressive strengthmeasurements should beat least five times larger than the maximum grain size ingraphite. As shown in Figure 1, 6 specimens were machinedfrom a MG pebble, in parallel orientation to the moldingdirection during fabrication. Five pebbles were used tomachine 30 cylindrical specimens. The machined specimenswere then heat-treated at 1900∘C in vacuum for 2 hoursto remove any trace of metallic impurities that might be

Table 1: Physical properties of specimens before oxidation.

Length (mm) Diameter (mm) Weight (g) Density(g/cm3)

Average 25.308 12.627 5.506 1.737St. dev. 0.033 0.023 0.032 0.004

introduced during machining. Then the specimens weresupersonically cleaned with acetone and ethanol and driedat 120∘C for 24 hours. Physical measurements showed thatthe specimens were uniform. The average dimension anddensity values with standard deviation of these specimensbefore oxidation are shown in Table 1.

2.2. Oxidation Measurements. The MG specimens were oxi-dized in air using the protocol recommended by ASTMD7542-09 [15]. A home-made experimental equipment buildat Oak Ridge National Laboratory (ORNL) was used forMG oxidation studies [8, 16]. Basically, the oxidation setupconsists of a three-zone vertical tube furnace and an analyticalbalance with weight-below port feature on the top of thefurnace. The graphite specimen is suspended by a Pt wirein the central zone, of uniform temperature, of the furnace.Theweight loss caused by oxidation is automatically recordedby computer in isothermal conditions. The oxidation tem-peratures at 500, 550, 600, 650, 700, 750, 800, 860, 900,and 950∘C were selected and at each temperature 2 or 3specimens were oxidized to around 15% weight loss with theair flow rate of 10 L/min to test repeatability. According to theASTMD7542-09, the oxidation rate at a given temperature isdetermined by a linear fit of the weight loss plotted againsttime in the range from 5% to 10% loss of original specimenweight. The activation energy (𝐸

𝑎) and preexponential factor

are calculated from the slope and intercept of the linearArrhenius plot of the logarithm of oxidation rate versus thereciprocal of absolute temperature [15, 16].

2.3. Testing of Compressive Strength. In order to reveal theeffect of oxidation on the mechanical properties of MG, thecompressive strengths of the specimens oxidized at two typ-ical temperatures were tested. The two selected temperaturesare 550∘C located in the kinetic regime and 900∘C in theboundary layer control regime, respectively. For comparison,the compressive strengths of pristine (unoxidized) MG spec-imens were measured as well. Because the MG specimensoxidized at 550∘C to around 15% weight loss were too weakto be handled in the strength test, specimens were oxidizedto approximately 10% weight loss before the compressivestrength test. Three specimens were tested for each oxidationcondition. An Instron Model 1322 Electromechanical TestSystem was used for performing the compressive strengthtests. AnMTS servohydraulic machine with a 25 kN load celland a 407 controller was used for performing the compressivestrength test at a crosshead speed of 0.00762mm/s usingLabView program. Irregular fracture behavior of the oxidizedspecimens may be caused by uncontrolled failure undercompression of the oxidized layer at the contact surfacebetween the parallel faces of the cylindrical specimen and the

Science and Technology of Nuclear Installations 3

X

Z

X

Y

1

2

1 & 2 3

4

5

6

3

5

∅60∅60

∅12.7

12.7

12.7

25.4

Unit: mmMolding direction

Figure 1: Schematic for the machining of specimens from a MG pebble.

loading fixtures. In order to avoid the uncontrolled failure, a1mm thick layer of oxidized layer was machined from bothparallel faces of all the oxidized cylindrical specimens, andthis surface preparation process does not reduce the strengthaccording to literature reported results [8].

2.4. Microstructure Characterizations. The MG specimens(both oxidized and pristine) were cut at 1/3 of the lengthfor scanning microscopy examination and test of Mercuryporosimetry. The shorter pieces were encased in resin andthen the exposed surface after cutting was polished usingseveral different grades of alumina powder (from coarse tofine) that allows for an optically flat surface. The microstruc-ture of MG specimens was investigated at 1500x magnifi-cation by Hitachi S-3000N Scanning Electron Microscope(SEM) system. The larger pieces were used in the MercuryPorosimetry to measure the porosity of MG. The volume ofthe pores in MG was determined by measuring the volumeof mercury intruding the MG at various pressures. Thevalue of the parameters applied in the Mercury Porosimetrymeasurement is as follows: the surface tension of themercury(𝛾) is 485 dynes/cm and the contact angle between mercuryand MG (𝜃) is 130.0∘, respectively.

3. Results and Discussions

The relationships between the residual weight and oxidationtime of MG at different temperatures in air were shown inFigure 2. It can be seen that the oxidizing temperature has aremarkable influence on the oxidation of MG. The oxidationrate accelerates significantly with the increasing temperaturewhen the oxidizing temperatures are below 800∘C. At 500∘C,it takes more than 200 hours to reach the weight loss of15%. When the oxidizing temperatures are 700∘C and 900∘C,the oxidation time for 15% weight loss is 0.9 and 0.2 hours,

0.1 1 10 100 10000.01Oxidation time (hour)

85

90

95

100

Resid

ual w

eigh

t (%

)

950∘C900∘C860∘C800∘C

750∘C700∘C650∘C

600∘C550∘C500∘C

650 600 550 500

Figure 2: The relationship between the weight loss and oxidationtime at different temperatures.

respectively. The average oxidation rate of MG at 900∘C ismore than 1000 times faster than that at 500∘C. However,with further increases in oxidizing temperature above 800∘C,the oxidation rates remain almost constant, showing thatthe oxidation control mechanisms at relatively low and highoxidizing temperatures are different.

Based on the average oxidation rate in the range from5% to 10% loss of original specimen weight, the Arrheniusplot of MG in the temperature range of 500–950∘C is shownin Figure 3. The slopes and intercepts of the best-fit linesfor three regimes were presented in Figure 3 as well. The


860

3000

500

500: oxidation T in Celsius degree21000: ratio of oxygen supply (moles/min)

to carbon consumption (moles/min)21000

43

27

14

R2 = 0.994

−3.5

−3.0

−2.5

−2.0

−1.5

−1.0

0.0

−0.5

lg(／

２m)

(Ｂ−1)

0.9 1.0 1.1 1.2 1.30.81000/T (＋−1)

y = −9.710x + 9.414

R2 = 0.985y = −4.753x + 4.358

R2 = 0.830y = −0.319x + 0.248

200

660650

600

550

700

800

950

900

750

Figure 3: Arrhenius plot of MG oxidized in air at 500–950∘C.

procedure recommended in ASTM D7542-09 was adoptedto calculate the 𝐸

𝑎for each regime from the Arrhenius plot.

The 𝐸𝑎of MG was determined to be 185.83 kJ/mol in the

chemical kinetics control regime, Regime I, at temperaturesbelow 700∘C; 90.96 kJ/mol in the in-pore controlled diffusionregime, Regime II, from 700 to 800∘C; and 6.10 kJ/mol inthe boundary layer control regime, Regime III, from 800 to950∘C. The 𝐸

𝑎in Regime II is around one-half of that in

Regime I and 𝐸𝑎in Regime III is very close to zero, in good

agreement with the 𝐸𝑎variation trend in the three regimes

mentioned elsewhere [11]. As the𝐸𝑎formost graphitemateri-

als was in the range of 190–210 kJ/mol [11], the slightly smaller𝐸𝑎for MG was probably due to the existence of incompletely

graphitized binder in the MG. The mass-normalized oxida-tion rate (OR

𝑚) for MG in Regime I can be described as

OR𝑚= 2.59 × 109 × exp(−22362.13𝑇 ) h−1, (1)

where 𝑇 is the temperature in Kelvin units.The ratio of oxygen supply rate (moles/min) to carbon

loss rate (moles/min) at relatively low temperatures is alsoshown in Figure 3. A previous report suggested that Arrhe-nius plots of standard size specimens (diameter = length =25.4mm) were linear as long as this ratio was larger than ∼10,indicating oxidation in Regime I [11]. For the MG specimenswith the diameter of 12.7mm, the plot was linear between 500and 700∘C. As the oxidation rate increased with the increasein temperature and the ratio declined gradually. When theratio dropped to ∼43 at 700∘C, the plot started to bend,which apparently does not agree with the rule mentionedabove. However, considering that the oxidation is uniformthroughout the graphite specimens in Regime I and the MGspecimens have only 1/4 mass of the standard specimens, thecarbon loss rate should be four times larger than the currentlyobserved rate if the standard size specimens were used withthe same air flow rate of 10 L/min. Accordingly, the ratioshould be reduced four times, from 43 to 11.75, which is veryclose to the empirically estimated threshold ratio of ∼10.

The average dimensional and corresponding densitychanges of specimens oxidized to∼10%weight loss at 550 and

Pristine

54.42

47.61

12.33

0

10

20

30

40

50

60

Com

pres

sive s

treng

th (M

Pa)

10% WL @900∘C 10% WL @550∘C

Figure 4: Average values for compressive strength ofMG specimensoxidized to ∼10% weight loss at 550 and 900∘C.

900∘C for compressive strength testing are shown in Table 2.The dimension did not change (within the experimentalerrors) after oxidation at 550∘C. However, the length anddiameter of specimens oxidized at 900∘C shrank by 1.3% and3.2%, respectively. As a result, the average density of the speci-mens oxidized at 550 and 900∘Cdeclined by 10.8% and 3.45%,respectively, showing the effect of different oxidation regimes.When the specimens were oxidized at 550∘C (in Regime I)the oxidation was uniform throughout the specimens, whichslightly changed their size but significantly decreased theirdensity. When the oxidation temperature increased to 900∘C(in Regime III), oxidation occurred mainly on the surface,which changed the dimensions but had only slight influenceon the density.

Figure 4 indicates the average values of compressivestrength for MG specimens oxidized to ∼10% weight loss at550 and 900∘C.With the comparative weight loss of∼10%, thecompressive strength of the specimens oxidized at 900∘C isabout four times larger than of those oxidized at 550∘C. Com-paring with the strength of pristine specimens, the reductionin compressive strength is 77.3% after oxidation at 550∘C inthe kinetic regime and only 12.5% for oxidation at 900∘C.Theaverage rate of strength loss for oxidation at 550∘C is 7.63% ofthe initial compressive strength value per each percentage ofweight loss. Meanwhile, after oxidation at 900∘C the figure isonly 1.17% of initial strength value for every 1% of weight loss.It is obvious that oxidation at 550∘Ccausesmore degradationsto compressive strength of MG than oxidation at 900∘C.Figures 5–7 show the SEM images of the pristine MG andMG specimens oxidized at 550∘C and 900∘C in air to about10% weight loss. Table 3 reports the value of porosity for MGspecimens and corresponding oxidation conditions. FromFigures 5 and 7, it can be seen that the microstructure differ-ence between the pristine MG and MG specimens oxidizedat 900∘C is too insignificant to be observed by eye. As shownin Table 3, the porosity of MG specimens oxidized at 900∘Cis slightly lower than that of the pristine MG, which is ingood agreement with the SEM investigation results. However,as shown in Figure 6, the microstructure of MG specimensoxidized at 550∘C dramatically changed after the oxidation

Science and Technology of Nuclear Installations 5

Table 2: Average physical property changes before and after oxidation to around 10% weight loss.

Oxidation 𝑇 (∘C) Weight loss (%) Length (%) Diameter (%) Density (%)Average St. dev. Average St. dev. Average St. dev. Average St. dev.

550 −10.13 0.35 +0.35 0.02 +0.20 0.01 −10.79 0.28900 −10.73 0.51 −1.29 0.03 −3.22 0.05 −3.45 0.12

Table 3: Porosity of MG specimens (both pristine and oxidized)measured by Mercury Porosimetry.

Samples Porosity (%)Pristine 18.09Oxidized at 550∘C to 10%WL 30.87Oxidized at 900∘C to 10%WL 19.74

Figure 5: SEM image of pristine A3-3 MG at 1500x Mag.

where a large amount of pores were developed and formed.Its porosity value increases remarkably from 18.09% of theporosity of pristineMG to 30.87%.The different developmentand formation of pores between the specimens oxidized at550 and 900∘C are due to the different oxidation mechanism:oxidation at 550∘C in the kinetic regime spreads uniformlyin the bulk, while oxidation at 900∘C in the boundary layercontrol regime is limited to a narrow surface layer and bulk isnot apparently affected. Comparing with the microstructureof the pristine MG shown in Figure 5, the binder materialof the MG specimens oxidized at 550∘C in Figure 6 shrinksor even disappears around the filler grains, which indicatesthe weaker oxidation resistance of the binder phase [10]. Ina previous report, it was found that the compressive strengthof PCEA graphite oxidized at 600∘C in the kinetic regime toaround 10% weight loss, dropping only by ∼26% [8]. Fromthemicrostructure images and porositymeasurement results,it can be concluded that the significant strength loss of theMG specimens oxidized at 550∘C in the kinetic regime maybe ascribed to both the pore formation throughout the bulkafter oxidation and the preferential binder phase oxidation.

4. Conclusion

Oxidation behavior of MG in air in the temperature rangefrom 500 to 950∘C was investigated. The activation energy

Figure 6: SEM image of A3-3 MG oxidized at 550∘C to 10% weightloss at 1500x Mag.

Figure 7: SEM image of A3-3 MG oxidized at 900∘C to 10% weightloss at 1500x Mag.

in the chemical kinetic control regime calculated from theArrhenius plot was ∼185 kJ/mol, slightly lower than that ofnuclear graphite, which indicates thatMGwasmore vulnera-ble to oxidation.Oxidizing temperature also played an impor-tant role in the degradation of compressive strength of MG.Oxidation at 900∘C caused less damage on the compressivestrength than oxidation at 550∘C. At the same weight lossof ∼10%, the compressive strength of specimens oxidized at900∘C was about 4 times larger than after oxidation at 550∘C.The average rates of strength loss for oxidation at 550∘Cand 900∘C are 7.63% and 1.17% of the initial compressivestrength value for every 1% of weight loss, respectively. Thesignificant strength loss of MG induced by the oxidation at550∘C in the kinetic regime was probably due to both theuniform pores formation throughout the bulk of MG and thepreferential oxidation of binder phase. This conclusion wassupported by the measurements on the compressive strengthand rationalized by the SEM examinations and porositymeasurements of the pristine and oxidized MG specimens.


Disclosure

This manuscript has been authored by UT-Battelle, LLCunder Contract no. DE-AC05-00OR22725, with the U.S.Department of Energy.TheUnited StatesGovernment retainsand the publisher, by accepting the article for publication,acknowledges that the United States Government retains anonexclusive, paid-up, irrevocable, worldwide license to pub-lish or reproduce the published form of this manuscript, orallowothers to do so, forUnited StatesGovernment purposes.TheDepartment of Energy will provide public access to theseresults of federally sponsored research in accordance withthe DOE Public Access Plan (https://energy.gov/downloads/doe-public-access-plan).

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

Supports from the State Scholarship Foundation of China(201406215002), the Chinese National S&T Major Project(ZX06901), Chinese National Natural Science Founda-tion (51420105006), and Key Program for InternationalS&T Cooperation Projects of China (2016YFE0100700) areacknowledged. The work performed at Oak Ridge NationalLaboratory was supported partially by the Advanced ReactorTechnologies program of U.S. Department of Energy, Officeof Nuclear Energy.

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